Weathering and Erosion

Earth’s surface is constantly changing and evolving. Features change over time due to weathering and erosion.

Weathering

The process by which rock and other substances at earth’s surface are broken down.

I. 2 main types of weathering-

A. Mechanical or physical weathering-

breaks rocks into pieces by physical means. Slow process can wear away whole mountains over time.

Types of mechanical weathering

a. freezing and thawing (frost action or ice wedging)- repeated freezing and thawing of water in cracks. As water freezes it expands breaking rocks apart ex- potholes in winter

b. root action- plants grow between cracks in rocks and sidewalks. As plant grows roots spread out breaking apart rocks. Ex- weed in your driveway

c. abrasion- grinding away of rock by rock particles carried by water, wind, ice, or gravity. Ex- balancing rocks in the desert

d. release of pressure- when pressure that was holding rock layers together is removed by processes of erosion, release of pressure causes layers to peel off like layers of an onion.

B. Chemical Weathering-

Process that breaks down rocks through chemical change.

Types of Chemical weathering

1. Oxidation- reaction of oxygen with rocks containing iron

creates rust, occurs faster if water is present or humid environment.

2. Carbonation- reaction of carbon dioxide with water to make carbonic acid. Weathers marble and limestone- makes caves and sinkholes.

3. Hydration- water weathers rock by dissolving them. This is the most important cause of chemical weathering.

4. Lichens- produce weak acids that break down rocks.

5. Acid rain- pollution in the air from the burning of fossil fuels combines with water to form acid rain. Causes rapid weathering.

II. Weathering Rate-

How fast weathering occurs

A. Factors affecting weathering rate-

1.Type of rock or mineral composition-

The minerals in the rock determine rate.

If minerals dissolve easily, rock will weather fast

If minerals are resistant to dissolving then they will

weather slowly.

Permeable rocks weather much more quickly because they allow water thru them and can dissolve from the inside out.

2. Climate- both kinds of weathering occur faster in wet

climates. Chemical is faster in warm humid climates, like down south.

3. Surface area and particle size- the more surface area

that is exposed to weathering agents, the faster the rock will weather. Smaller particles weather faster than large ones.